In a conventional substation instrument control system, after a transformer (an instrument current transformer (“CT”) or an instrument voltage transformer (“VT”)) attached to a substation instrument main body converts a current or a voltage of a main circuit of a substation instrument into a current or a voltage suitable for measurement, a protection control system measures a waveform (hereinafter referred to as waveform signal) of the current or voltage. However, in conventional substation instrument control systems, it is necessary to connect an electric cable from the substation instrument main body to the protection control system for each waveform signal.
Another approach is to apply a process bus to the substation instrument control system. In the substation instrument control system, a merging unit is disposed near the substation instrument and the waveform signals input from the plural transformers are temporarily input to the merging unit. The merging unit digitizes and merges the waveform signals from the transformers, and transmits the digitized and merged waveform signal to a high-level protection control system (Intelligent Electronic Device (“IED”)) through a serial communication called process bus. Digitizing and transmitting the waveform signals through the process bus may eliminate the electric cables, reduce a secondary-side load of the transformer, and standardize the protection control system.
The serial communication through the process bus of the substation instrument control system may be standardized to facilitate communication between the merging unit and the IED, even in instances when the merging unit and the IED are made by different manufacturers. However, the type of substation instrument used (e.g., voltage transformer or power transmission line), the type of IED used (e.g., a protection device or a control system), the disposition of the substation instrument or the IED, and required performance/characteristics of the substation instrument or the IED may depend on the substation. Accordingly, the substation characteristics may affect the setting of the merging unit.
The setting of the merging unit may include the sampling rate of the waveform signal output from the transformer, the maximum value (full scale) of the waveform signal that can be input, and the filter characteristic of an analog Low-Pass Filter (“LPF”), which is necessary to prevent aliasing generated in digitally converting the waveform signal.
It is not practical to achieve standardization by preparing merging unit hardware for the numerous combinations of characteristics, as doing so would require a significant number of different merging unit types. Moreover, the disposition and configuration of the substation instruments typically varies by expansion. It is, therefore, desirable to be able to easily change the merging unit settings onsite.